Kohji Toda

Bio: Kohji Toda is an academic researcher from Tokyo City University. The author has contributed to research in topics: Interdigital transducer & Lamb waves. The author has an hindex of 12, co-authored 169 publications receiving 677 citations.

Acoustic wave characteristics of lead-free (Bi1/2Na1/2)0.99Ca0.01TiO3piezoelectric ceramic

Abstract: Acoustic wave characteristics of lead-free (Bi1/2Na1/2)0.99Ca0.01TiO3 (BNC-1) piezoelectric ceramic are studied from a viewpoint of a new substrate material group for a surface acoustic wave and/or a bulk acoustic wave. The BNC-1 ceramic, which has high electromechanical coupling factors, k33(=37.6%) and kt(=51.3%), and low free permittivities, e33T/e0(=236) and e11T/e0(=271), along with a high mechanical strength, is favorable to high frequency ultrasonic uses. Two interdigital transducers can excite an SH plate mode wave or a Lamb mode wave whose travelling direction is perpendicular or parallel to the poling axis, respectively. A delay line type oscillator using the SH wave is demonstrated a single mode oscillation of 8.319 MHz.

Piezoelectric Properties of Bismuth Layer-Structured Ferroelectric Na0.5Bi4.5Ti4O15 Ceramic

Abstract: Piezoelectric properties of the grain-oriented Na0.5Bi4.5Ti4O15 (NBT) ceramic, belong to the bismuth layerstructured ferroelectric (BLSF) ceramics are studied from applicational viewpoints on grain orientation prepared by the hot-forging (H.F.) method. The MnCO3 (0.1 wt%) doped NBT (NBT+Mn(0.1)) ceramic can stand the high temperature poling process, because of the high resistivity. The grain orientation (H.F.) makes the piezoelectric properties of NBT+Mn(0.1) ceramic much more pronounced than those of the non-oriented (O.F.) ceramic. Coupling factors k33 and k15, along with piezoelectric constants d33, d15, g33 and g15, of the H. F. NBT+Mn(0.1) ceramic are enhanced, and their anisotropies in k33/k31 and k15/k24 are emphasized. A bulk wave delay line oscillator using the ceramic substrate is demonstrated.

Determination of the crystal structure of Pb2CrO5

Abstract: The crystal structure analysis of Pb2CrO5 is determined by comparison with powder x‐ray and electron diffraction data. The structure is monoclinic: c2/m. All of the atom positions are determined by considering the calculated values of d spacings, line intensities, bond lengths, and bond angles. A new set of lattice parameters are a=14.018, b=5.683, c=7.143 A, and β=115.23 °. A preliminary result on photoconductivity in a Pb2CrO5 ceramic disk is also given.

Analysis of lamb wave propagation characteristics in rotated Y-cut X-propagation LiNbO3 plates

Abstract: Lamb wave propagation characteristics in rotated Y-cut X-propagation LiNbO3 plates are analyzed theoretically as a parameter of the rotation angle of the crystal to elucidate velocity dispersion characteristics, mechanical displacement and electric potential, and electromechanical coupling constants. Calculation results show Lamb wave propagation characteristics in the 90° rotated Z-cut plate and the 128° rotated Y-cut plate. The zero-order symmetrical mode at a 0° rotation angle (Y-cut) has a high coupling constant over 20 percent. The zero-order symmetrical and antisymmetrical modes degenerate with a plate thickness more than three wavelengths and become the Rayleigh wave range in substance. In measurement of the central frequency of the Lamb wave filter, the experimental results agree well with the theoretical analysis and the analytical results by this method are useful in constructing Lamb wave devices using interdigital transducers.

Evaluation of nematic liquid-crystal director-orientation using shear horizontal wave propagation

Abstract: The nematic liquid-crystal director-orientation in the vicinity of the interface between a liquid-crystal layer and a glass substrate is evaluated in various orientations under an applied electric field, using shear horizontal (SH) wave propagation. The measured acoustic phase delay is related to numerical analysis of the propagation characteristics of the SH wave in a trilayer structure containing a nematic liquid-crystal layer (4-cyano-4′-pentylbiphenyl: 5CB) between two glass plates. This technique using SH wave propagation in a liquid-crystal cell is promising for evaluating the nematic liquid-crystal director angle in the vicinity of the interface.

Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics

Abstract: Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics are reviewed with the aim of providing an insight into different processes which may affect the behaviour of ferroelectric devices, such as ferroelectric memories and micro-electro-mechanical systems. Taking into consideration recent advances in this field, topics such as polarization switching, polarization fatigue, effects of defects, depletion layers, and depolarization fields on hysteresis loop behaviour, and contributions of domain-wall displacement to dielectric and piezoelectric properties are discussed. An introduction into dielectric, pyroelectric, piezoelectric and elastic properties of ferroelectric materials, symmetry considerations, coupling of electro-mechanical and thermal properties, and definitions of relevant ferroelectric phenomena are provided.

Transferring lead-free piezoelectric ceramics into application

Abstract: After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Lead Free Piezoelectric Materials

Abstract: Lead oxide based ferroelectrics, represented by lead zirconate titanate (Pb(Zr, Ti)O3) or PZT) are the most widely used materials for piezoelectric actuators, sensors and transducers due to their excellent piezoelectric properties. Considering lead toxicity, there is interest in developing piezoelectric materials that are biocompatible and environmentally friendlier. The low density of non-lead based materials can also be an advantage in transducers for underwater and medical imaging due to expected lower acoustical impedance. Another impetus for seeking alternative to lead based compositions is the need for piezoelectric materials for operation at high temperatures. Several classes of materials are now being reconsidered as potentially attractive alternatives to PZT for special applications. The potassium niobate family, KNbO3, exhibits low dielectric constants, large thickness coupling coefficient along certain non-polar directions, and low density, all of which have advantages for high frequency transducer applications. Several compositions belonging to bismuth titanate family, Bi4Ti3O12, such as SrTi4Bi4O15, are promising candidates for high temperature applications. Lead free materials alone (eg. (Na0.5Bi0.5)TiO3) or in solution with PT (BiScO3 – PbTiO3) are also potentially interesting as they combine high piezoelectric activity and, in some cases, relatively high T c . For these families of piezoelectric materials, the processing and piezoelectric response under different conditions of pressure, frequency, and temperature are presently much less understood than for the classical lead containing systems. In this presentation we review and discuss piezoelectric properties of selected lead free compositions (principally for members of the potassium niobate family and bismuth titanate layered compounds) in relation to structural and microstructural features as well as extrinsic contributions (domain walls displacement, conductivity) to their electromechanical properties. It is shown that it is possible to obtain remarkably stable piezoelectric response in some compositions, while others exhibit strong dependence of piezoelectric properties on driving field and frequency. Origins of these different behaviours are discussed.

Processing and Electromechanical Properties of (Bi0.5Na0.5)(1−1.5x)LaxTiO3 Ceramics

Abstract: Bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) with 0–6 at.% lanthanum was prepared by the conventional mixed oxide method. Each composition was calcined at 800–900°C for 2–5 h to form a pure perovskite phase. Green pellets were sintered at 1050–1150°C for 1–4 h to obtain dense ceramics with at least 95% of theoretical density. X–ray diffraction (XRD) showed phase distortion as lanthanum was added to this system. Meanwhile, a small amount of La was found to affect the grain size and had an influence on the poling conditions and electrical properties. The BNT–based composition with 1 at.% La doping provided a dielectric constant (K) of 560, a piezoelectric charge constant (d33) of 92 pC/N, and a hydrostatic piezoelectric coefficient (dh) of 72 pC/N.

Mechanism of operation and design considerations for surface acoustic wave device vapor sensors

Abstract: Abstract Surface acoustic wave (SAW) devices offer many attractive features for application as vapour phase chemical microsensors. This paper describes the characteristics of SAW devices and techniques by which they can be employed as vapour sensors. The perturbation of SAW velocity by polymeric coating films is investigated both theoretically and experimentally. Highest sensitivity can be achieved when the device is used as the resonating element in a delay line oscillator circuit. A simple equation has been developed from theoretical considerations which offers reasonably accurate quantitative predictions of SAW device frequency shifts when subjected to a given mass loading. In this mode the SAW device behaves very like conventional bulk-wave quartz crystal microbalances except that the sensitivity can be several orders of magnitude higher and the device size can be several orders of magnitude smaller. Detection of mass changes of a few femtograms by a SAW device having a surface area of 10−4 cm2 is theoretically possible.